Myocardial contraction

Question 1

Myocardium main components

Answer 1

Contractile tissue.
Connective tissue.
Fibrous frame.
Specialised conduction system.

Question 2

Interactions for pumping action

Answer 2

Depends on interactions between the contractile proteins and its muscular walls.
The interactions transform chemical energy from ATP to mechanical - moves blood under pressure from great veins to PA and from PV to the aorta.

Question 3

Contractile proteins

Answer 3

Activated by a signalling process called excitation-contraction coupling.

Question 4

Excitation-contraction coupling

Answer 4

Begins when the action potential depolarizes the cell and ends when ionized calcium (appears within the cytosol) binds to the calcium receptor of the contractile apparatus.
Movement of calcium into cytosol - passive process mediated by calcium channels.

Question 5

Heart relaxing

Answer 5

The heart relaxes when ion exchangers and pumps - transport calcium uphill - out of the cytosol.

Question 6

Working myocardial cell

Answer 6

Filled with cross-striated myofibrils.
Plasma membrane regulates excitation-contraction coupling and relaxation - also separates the cytosol from extra-cellular space and sarcoplasmic reticulum.
Mitochondria - ATP.

Question 7

Myocardial metabolism

Answer 7

Relies on free fatty acids - during aerobic.
During hypoxia - no FFA metabolism - anaerobic then used.

Question 8

Ultra-structure of myocardial

Answer 8

Contractile proteins - in regular array of thick and thin filaments.
A band - region of sarcomere - thick filaments.
I band - thin filaments - extend towards the centre of the sarcomere from the Z lines - contains tropomyosin and troponin.
Z lines - bisect each I band.

Question 9

Sarcomere

Answer 9

The sarcomere - functional unit of contractile apparatus - defined as the region between a pair of Z lines - contains 2 half I bands and 1 A band.

Question 10

Sarcoplasmic reticulum

Answer 10

Membrane network - surrounds contractile proteins.
Consists of the sarcotubular network at the centre of the sarcomere and the subsarcolemmal cisternae - touch the T-tubules and sarcolemma.

Question 11

T-tubule

Answer 11

The transverse tubular system.
Lined by a membrane that is continuous with the sarcolemma.
The lumen of the T-tubules carry the extracellular space toward the centre of the myocardial cell.

Question 12

Contraction

Answer 12

Sliding of actin over myosin by ATP hydrolysis by ATPase in the myosin head.
These heads form crossbridges that interact with actin - after linkage between calcium and TnC - and deactivation of tropomyosin and TnI.

Question 13

Myosin

Answer 13

2 heavy chains.
4 light chains.
Heads are perpendicular on the thick filament at rest - bend towards centre during contraction.
alpha and beta myosin

Question 14

Actin

Answer 14

Globular protein.
Double-stranded macromolecular helix.
Both form the F actin.

Question 15

Tropomyosin

Answer 15

Elongated molecule - 2 helical peptide chains.
Occupies each of the longitudinal grooves between the two actin strands.
Regulated interaction of the other three.

Question 16

Troponin

Answer 16

I - with tropomyosin inhibit actin and myosin interaction.
T - binds troponin complex to tropomyosin.
C - high affinity calcium binding sites, signalling contraction.

Question 17

Control of the contractile cycle

Answer 17

Calcium ions.
Troponin phosphorylation.
Myosin ATPase.